1. Field of the Invention
This invention relates to a system which will discriminate between valid transitions from one level to the other level of a two level logic signal, and those transitions caused by noise. The system uses digital circuits to reject short time duration input pulses which represent noise and to accept as valid only those input signal transitions which exist for at least a predetermined time duration. Once a valid transition from a first to a second level is sensed, the system changes its state, generates an output signal which indicates that a valid transition has occurred, and thereupon functions to discriminate between transitions from the second to the first level.
2. Description of the Prior Art
There are numerous electronic circuits which will produce an output signal only when an input signal is present for a predetermined time period. Many such prior art circuits use counters or integrators to determine the time duration of the input signal. It has been found, however, that many of the prior art circuits are very sensitive to high frequency noise which is present in the input signal, and will not function properly in a noisy environment. The prior art circuits, in addition, are generally sensitive only to input signals of a predetermined polarity or direction, and will not respond to changes in both directions between two signal levels as commonly occur in digital logic networks.
Similar deficiencies occur in the prior art circuits which are designed to eliminate or discriminate against contact bounce or chatter. Many of these circuits will function properly to eliminate the noise that occurs upon the actuation of a switch, but will not function properly if noise occurs shortly thereafter.
A particular problem occurs in digital logic circuits where transitions occur in both directions between two discrete levels. For example, to discriminate between both the opening and closing of a switch, three signal lines per switch are required with the prior art circuits. If the switch is in a noisy environment, such as the points in an automobile ignition system, additional circuitry is required to filter or eliminate the noise in the signal and to validly discriminate between opening and closing of the switch. In some environments, additionally, conventional analog filters cannot be employed without loading down or sacrificing the noise immunity of the digital logic circuits.